Photoelectric conversion element and solar cell

ABSTRACT

A photoelectric conversion element is provided. The photoelectric conversion element comprises a substrate, a first electrode, an electron transport layer, a hole transport layer, and a second electrode. The electron transport layer comprises a photosensitizing compound. The hole transport layer comprises a basic compound A and an ionic compound B. The basic compound A is represented by the following formula (1): 
     
       
         
         
             
             
         
       
     
     where each of R 1  and R 2  independently represents an alkyl group or an aromatic hydrocarbon group, or R 1  and R 2  share bond connectivity to form a nitrogen-containing heterocyclic ring; and the ionic compound B is represented by the following formula (2): 
     
       
         
         
             
             
         
       
     
     where X +  represents a counter cation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is based on and claims priority pursuant to 35U.S.C. § 119(a) to Japanese Patent Application No. 2017-001988, filed onJan. 10, 2017 in the Japan Patent Office, the entire disclosure of whichis hereby incorporated by reference herein.

BACKGROUND Technical Field

The present disclosure relates to a photoelectric conversion element anda solar cell.

Description of the Related Art

In recent years, the driving power for an electronic circuit has becomeextremely small. It is now possible to drive various types of electroniccomponents, such as sensors, with weak electric power. For utilizingsensors, stand-alone power sources (environmental power generatingelements) that can generate and consume power on the spot are expected.In particular, solar cells are attracting attention as elements that cangenerate power anywhere there is light.

Among the solar cells, it is known that solid-state dye-sensitized solarcells remarkably degrade power generating ability under high-temperatureenvironments due to crystallization of the hole transport layer. Inattempting to solve this problem, one proposed approach involvessuppressing the crystallization by increasing steric hindrance by, forexample, introducing an alkyl group to the molecular backbone of thehole transport material. On the other hand, it has been reported thatcurrent loss due to internal resistance of a photoelectric conversionelement is remarkable when weak light, such as indoor light, isconverted to electricity.

Heat resistance of such solid-state dye-sensitized solar cells has beenevaluated under pseudo sunlight so far. That evaluated under weak light,such as room light, has never been reported. Stand-alone power sourceswill be more demanded in the future, for sensors installed inhigh-temperature environments where no one will step in. A powergenerating element for driving electronic components needs to generate acertain level of voltage. A reduction of the generated power (voltage)may cause defective driving of the electronic components.

SUMMARY

In accordance with some embodiments of the present invention, aphotoelectric conversion element is provided. The photoelectricconversion element comprises a substrate, a first electrode, an electrontransport layer, a hole transport layer, and a second electrode. Theelectron transport layer comprises a photosensitizing compound. The holetransport layer comprises a basic compound A and an ionic compound B.The basic compound A is represented by the following formula (1):

where each of R₁ and R₂ independently represents an alkyl group or anaromatic hydrocarbon group, or R₁ and R₂ share bond connectivity to forma nitrogen-containing heterocyclic ring; and the ionic compound B isrepresented by the following formula (2):

where X⁺ represents a counter cation.

In accordance with some embodiments of the present invention, a solarcell is provided. The solar cell comprises the above photoelectricconversion element.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a cross-sectional view of a photoelectric conversion elementin accordance with some embodiments of the present invention; and

FIG. 2 is a schematic diagram of a secondary battery charging circuitused in Examples.

The accompanying drawings are intended to depict example embodiments ofthe present invention and should not be interpreted to limit the scopethereof. The accompanying drawings are not to be considered as drawn toscale unless explicitly noted.

DETAILED DESCRIPTION

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the presentinvention. As used herein, the singular forms “a”, “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. It will be further understood that the terms“includes” and/or “including”, when used in this specification, specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

Embodiments of the present invention are described in detail below withreference to accompanying drawings. In describing embodimentsillustrated in the drawings, specific terminology is employed for thesake of clarity. However, the disclosure of this patent specification isnot intended to be limited to the specific terminology so selected, andit is to be understood that each specific element includes all technicalequivalents that have a similar function, operate in a similar manner,and achieve a similar result.

For the sake of simplicity, the same reference number will be given toidentical constituent elements such as parts and materials having thesame functions and redundant descriptions thereof omitted unlessotherwise stated.

In accordance with some embodiments of the present invention, aphotoelectric conversion element is provided that is capable ofsuppressing voltage reduction and producing high power output even underhigh-temperature and weak-light environments.

The photoelectric conversion element in accordance with some embodimentsof the present invention comprises a substrate, a first electrode, anelectron transport layer comprising, a hole transport layer, and asecond electrode. The electron transport layer comprises aphotosensitizing compound. The hole transport layer comprises a basiccompound A represented by the following formula (1) and an ioniccompound B represented by the following formula (2).

In the formula (1), each of R₁ and R₂ independently represents an alkylgroup or an aromatic hydrocarbon group, or R₁ and R₂ share bondconnectivity to form a nitrogen-containing heterocyclic ring.

In the formula (2), X⁺ represents a counter cation.

By using the combination of the basic compound A and the ionic compoundB, the photoelectric conversion element can suppress voltage reductionand produce high power output even under high-temperature environments.The high-temperature environments may refer to, for example,environments having a temperature of 40° C. or higher.

The photoelectric conversion element can suppress voltage reduction andproduce high power output even under weak light such as indoor light.

The photoelectric conversion element and solar cell in accordance withsome embodiments of the present invention are described below withreference to FIG. 1. FIG. 1 is a cross-sectional view of a photoelectricconversion element in accordance with some embodiments of the presentinvention.

Referring to FIG. 1, a first electrode 2 is formed on a substrate 1. Ahole blocking layer 3 is formed on the first electrode 2. An electrontransport layer 4, containing a photosensitizing compound 5, is formedon the hole blocking layer 3. A second electrode 7 is disposed facingthe first electrode 2, and a hole transport layer 6 is disposedtherebetween. In addition, lead lines 8 and 9 for electricallyconnecting the first electrode 2 and the second electrode 7 areprovided.

Substrate

The substrate 1 is not limited to any particular material. Preferably,the substrate 1 is made of a transparent material, such as a glassplate, a transparent plastic plate, a transparent plastic film, and aninorganic transparent crystalline body.

First Electrode

The first electrode 2 is made of a visible-light-transmissive conductivematerial. Examples of the visible-light-transmissive conductive materialinclude those conventionally used for photoelectric conversion elementsand liquid crystal panels.

Specific examples of such materials used for the first electrode 2include, but are not limited to, indium-tin oxide (ITO), fluorine-dopedtin oxide (FTO), antimony-doped tin oxide (ATO), indium-zinc oxide,niobium-titanium oxide, and graphene. Each of these substances may beused alone to form a single layer or in combination with others to forma multilayer.

Preferably, the first electrode has a thickness of from 5 nm to 10 μm,and more preferably from 50 nm to 1 μm.

To maintain a constant level of rigidity, the first electrode 2 ispreferably formed on the substrate 1 made of avisible-light-transmissive material such as a glass plate, a transparentplastic plate, a transparent plastic film, and an inorganic transparentcrystalline body.

A combined body of the first electrode 2 and the substrate 1 may also beused. Examples of such a combined body include, but are not limited to,an FTO-coated glass plate, an ITO-coated glass plate, azinc-oxide-and-aluminum-coated glass plate, an FTO-coated transparentplastic film, and an ITO-coated transparent plastic film.

In addition, a combined body of a substrate (such as glass substrate)with a transparent electrode made of tin oxide or indium oxide dopedwith a cation or anion having a different atomic valence, or with ametallic electrode having a mesh-like or stripe-like structure to belight transmissive, can also be used.

Each of these materials can be used alone, or mixed with or laminated onthe others.

For the purpose of reducing resistance, metallic lead wires may be usedin combination. The metallic lead wire may be made of aluminum, copper,silver, gold, platinum, or nickel. The metallic lead wire may bedisposed to the substrate by means of vapor deposition, sputtering, orpressure bonding, and ITO or FTO may be further disposed thereon.

Hole Blocking Layer

The hole blocking layer 3 is preferably made of avisible-light-transmissive material. Examples of such a material includean electron transport material. Preferred examples of the electrontransport material include titanium oxide. The hole blocking layer 3 isprovided for suppressing voltage reduction that is caused when a hole inan electrolyte and an electron on the surface of the electrode arerecombined (i.e., reverse electron transfer occurs) as the electrolytecomes into contact with the electrode. The effect of the hole blockinglayer 3 is remarkably exerted especially in a solid-state dye-sensitizedsolar cell. This is because, in a solid-state dye-sensitized solar cellgenerally containing an organic hole transport material, therecombination (reverse electron transfer) speed of a hole in the holetransport material with an electron on the surface of the electrode isgreater than that in a wet-state dye-sensitized solar cell containing anelectrolytic solution.

The hole blocking layer 3 is preferably formed by a method that canimpart a high internal resistance to the resulting hole blocking layerso as to suppress current loss under indoor light, but the formingmethod is not limited thereto. Generally, the hole blocking layer can beformed by a sol-gel method that is one of wet film-forming methods.However, the film formed by this method cannot sufficiently suppresscurrent loss because the film density is low. On the other hand, thefilm formed by sputtering, that is one of dry film-forming methods, cansuppress current loss since the film density is sufficiently high.

The hole blocking layer 3 has another function of preventing the firstelectrode 2 and the hole transport layer 6 from electrically contactingwith each other. Preferably, the thickness of the hole blocking layer 3is from 5 nm to 1 μm, but is not limited thereto. When the hole blockinglayer is formed by a wet film-forming method, the preferred thickness isfrom 500 to 700 nm. When the hole blocking layer is formed by a dryfilm-forming method, the preferred thickness is from 10 to 30 nm.

Electron Transport Layer

The electron transport layer 4 contains the photosensitizing compound 5.The electron transport layer 4 may have a configuration in which thephotosensitizing compound 5 is adsorbed to an electron transportmaterial. In the present embodiment, the electron transport layer 4 isporous and formed on the hole blocking layer 3. The electron transportlayer 4 may be either single-layered or multi-layered.

In the latter case, multiple dispersion liquids containing semiconductorparticles of different particle diameters may be applied multiply, ormultiple layers of different kinds of semiconductors or differentcompositions of resins and additives may be applied multiply. In a casein which the thickness is insufficient as a result of singleapplication, multiple application is effective.

Generally, as the thickness of the electron transport layer 4 increases,the amount of photosensitizing materials carried per unit projected areaincreases, and therefore the light capture rate increases. However, atthe same time, the diffusion distance of injected electrons alsoincreases, thus increasing loss due to recombination of charge.

Accordingly, the thickness of the electron transport layer 4 ispreferably in the range of from 100 nm to 100 μm.

In a case in which a semiconductor is used as the electron transportmaterial, the semiconductor is not limited to any particular material.Specific examples of the semiconductor include, but are not limited to,single-body semiconductors such as silicon and germanium, compoundsemiconductors such as metal chalcogenides, and compounds having aperovskite structure.

Specific examples of the metal chalcogenides include, but are notlimited to, oxides of titanium, tin, zinc, iron, tungsten, zirconium,hafnium, strontium, indium, cerium, yttrium, lanthanum, vanadium,niobium, and tantalum; sulfides of cadmium, zinc, lead, silver,antimony, and bismuth; selenides of cadmium and lead; and tellurides ofcadmium.

Specific examples of the compound semiconductors include, but are notlimited to, phosphides of zinc, gallium, indium, and cadmium; galliumarsenide; copper-indium selenide; and copper-indium sulfide.

Specific examples of the compounds having a perovskite structureinclude, but are not limited to, strontium titanate, calcium titanate,sodium titanate, barium titanate, and potassium niobate.

Among these materials, oxide semiconductors are preferable, and titaniumoxide, zinc oxide, tin oxide, and niobium oxide are more preferable.Each of these materials can be used alone or in combination with others.The semiconductor is not limited in crystal type and may be eithersingle crystalline, polycrystalline, or amorphous.

In a case in which semiconductor particles are used as the electrontransport material, the semiconductor particles are not limited in size.Preferably, the average particle diameter of the primary particlethereof is in the range of from 1 to 100 nm, more preferably from 5 to50 nm.

It is possible to further improve efficiency by mixing or stackinganother type of semiconductor particles having a greater averageparticle diameter so that incident light can scatter. In this case, theaverage particle diameter of the semiconductor particles is preferablyin the range of from 50 to 500 nm.

The electron transport layer 4 is not limited in its formation methodand can be formed by, for example, a vacuum film-forming method, such assputtering, or a wet film-forming method. For reducing production cost,wet film-forming methods are preferable. Specifically, a method ofapplying a paste dispersing a powder or sol of semiconductor particlesonto the hole blocking layer 3, disposed on the first electrode 2serving as an electron collecting electrode substrate, is preferable.

In wet film-forming methods, how to apply the paste is not particularlylimited. For example, the paste may be applied by means of dipping,spraying, wire bar, spin coating, roller coating, blade coating, gravurecoating, or wet printing such as relief, offset, gravure, intaglio,rubber plate, and screen printings.

A dispersion liquid of semiconductor particles may be prepared by meansof mechanical pulverization or mill, specifically by dispersing at leastthe semiconductor particles alone or a mixture of the semiconductorparticles with a resin in water or an organic solvent. Specific examplesof the resin mixed with the semiconductor particles include, but are notlimited to, homopolymers and copolymers of vinyl compounds such asstyrene, vinyl acetate, acrylate, and methacrylate; and silicone resin,phenoxy resin, polysulfone resin, polyvinyl butyral resin, polyvinylformal resin, polyester resin, cellulose ester resin, cellulose etherresin, urethane resin, phenol resin, epoxy resin, polycarbonate resin,polyarylate resin, polyamide resin, and polyimide resin.

Specific examples of solvents for dispersing the semiconductor particlesinclude, but are not limited to, water; alcohol solvents such asmethanol, ethanol, isopropyl alcohol, and α-terpineol; ketone solventssuch as acetone, methyl ethyl ketone, and methyl isobutyl ketone; estersolvents such as ethyl formate, ethyl acetate, n-butyl acetate; ethersolvents such as diethyl ether, dimethoxyethane, tetrahydrofuran,dioxolan, and dioxane; amide solvents such as N,N-dimethylformamide,N,N-dimethylacetamide, and N-methyl-2-pyrrolidone; halogenatedhydrocarbon solvents such as dichloromethane, chloroform, bromoform,methyl iodide, dichloroethane, trichloroethane, trichloroethylene,chlorobenzene, o-dichlorobenzene, fluorobenzene, bromobenzene,iodobenzene, and 1-chloronaphthalene; and hydrocarbon solvents such asn-pentane, n-hexane, n-octane, 1,5-hexadiene, cyclohexane,methylcyclohexane, cyclohexadiene, benzene, toluene, o-xylene, m-xylene,p-xylene, ethylbenzene, and cumene. These solvents can be used alone orin combination as a mixed solvent.

To prevent reaggregation of particles in the dispersion liquid ofsemiconductor particles or the paste of semiconductor particles obtainedby sol-gel method, etc., an acid (e.g., hydrochloric acid, nitric acid,and acetic acid), a surfactant (e.g., polyoxyethylene(10) octyl phenylether), or a chelator (e.g., acetyl acetone, 2-aminoethanol, andethylenediamine) may be added thereto.

To improve film-forming performance, a thickener can also be added.

Specific examples of the thickener include, but are not limited to,polymers such as polyethylene glycol and polyvinyl alcohol, and ethylcellulose.

It is preferable that semiconductor particles having been applied arebrought into electronic contact with each other and exposed to burning,microwave irradiation, electron beam irradiation, or laser lightirradiation, for increasing the film strength and adhesion to thesubstrate. Each of these treatments can be conducted alone or incombination with others.

In the burning, the burning temperature is preferably in the range offrom 30° C. to 700° C., more preferably from 100° C. to 600° C., but isnot limited thereto. When the burning temperature is excessively raised,the resistance of the substrate may become too high or the substrate maymelt. The burning time is preferably from 10 minutes to 10 hours, but isnot limited thereto.

In the microwave irradiation, the electron transport layer may beirradiated from either the layer-formed side or the opposite sidethereof. The irradiation time is preferably within 1 hour, but is notlimited thereto.

After the burning, for the purpose of increasing the surface area of thesemiconductor particles as well as increasing the efficiency of electroninjection from the photosensitizing compound to the semiconductorparticles, a chemical plating treatment using an aqueous solution oftitanium tetrachloride or a mixed solution thereof with an organicsolvent, or an electrochemical plating treatment using an aqueoussolution of titanium trichloride may be conducted.

A layer in which semiconductor particles having a diameter of severaltens nanometers are stacked by sintering, etc., forms a porousstructure. Such a nano porous structure has a very large surface area.The surface area can be represented by a roughness factor. The roughnessfactor is a numerical value indicating the ratio of the actual area ofthe inside of the porous structure to the surface area of thesemiconductor particles applied to the substrate. Accordingly, thehigher the roughness factor, the better. In view of the thickness of theelectron transport layer, the roughness factor is preferably 20 or more.

Photosensitizing Compound

In the present embodiment, for more improving conversion efficiency, thephotosensitizing compound 5 (hereinafter may be referred to as“photosensitizing material”) is adsorbed to the surface of the electrontransport material contained in the electron transport layer 4.

The photosensitizing material is not limited to any particular compoundso long as it is capable of being photoexcited. Specific examples ofsuch materials include, but are not limited to, the following compounds:

metal complex compounds described in JP-07-500630-A, JP-10-233238-A,JP-2000-26487-A, JP-2000-323191-A, and JP-2001-59062; coumarin compoundsdescribed in JP-10-93118-A, JP-2002-164089-A, JP-2004-95450-A, and J.Phys. Chem. C., 7224, Vol. 111 (2007); polyene compounds described inJP-2004-95450 and Chem. Commun., 4887 (2007); indoline compoundsdescribed in JP-2003-264010-A, JP-2004-63274-A, JP-2004-115636-A,JP-2004-200068-A, JP-2004-235052-A, J. Am. Chem. Soc., 12218, Vol. 126(2004), Chem. Commum., 3036 (2003), and Angew. Chem. Int. Ed., 1923,Vol. 47 (2008); thiophene compounds described in J. Am. Chem. Soc.,16701, Vol. 128 (2006) and J. Am. Chem. Soc., 14256, Vol. 128 (2006);cyanine dyes described in JP-11-86916-A, JP-11-214730-A,JP-2000-106224-A, JP-2001-76773-A, and JP-2003-7359-A; merocyanine dyesdescribed in JP-11-214731-A, JP-11-238905-A, JP-2001-52766-A,JP-2001-76775-A, and JP-2003-7360-A; 9-aryl xanthene compounds describedin JP-10-92477-A, JP-11-273754-A, JP-11-273755-A, and JP-2003-31273-A;triarylmethane compounds described in JP-10-93118-A and JP-2003-31273-A;and phthalocyanine compounds and porphyrin compounds described inJP-09-199744-A, JP-10-233238-A, JP-11-204821-A, JP-11-265738-A, J. Phys.Chem., 2342, Vol. 91 (1987), J. Phys. Chem. B, 6272, Viol. 97 (1993),Electroanal. Chem., 31, Vol. 537 (2002), JP-2006-032260-A, J. PorphyrinsPhthalocyanines, 230, Vol. 3 (1999), Angew. Chem. Int. Ed., 373, Vol. 46(2007), and Langmuir, 5436, Vol. 24 (2008).

Among these compounds, metal complex compounds, coumarin compounds,polyene compounds, indoline compounds, and thiophene compounds arepreferable. More specifically, compounds available from Mitsubishi PaperMills Limited under the product names D131, D102, and D358, respectivelyrepresented by the following formulae (i), (ii), and (iii), arepreferable.

The photosensitizing material can be adsorbed to the electron transportmaterial contained in the electron transport layer 4 by dipping theelectron collecting electrode (i.e., the first electrode 2) having theelectron transport layer 4 thereon in a solution or dispersion of thephotosensitizing material, or applying the solution or dispersion of thephotosensitizing material to the electron transport layer 4.

In the former case, for example, an immersion method, a dipping method,a roller method, or an air knife method may be employed.

In the latter case, for example, a wire bar method, a slide hoppermethod, an extrusion method, a curtain method, a spin method, or a spraymethod may be employed.

Alternatively, the adsorption can be performed in a supercritical fluidof carbon dioxide etc.

When adsorbing the photosensitizing material to the electron transportmaterial, a condensation agent can be used in combination.

The condensation agent may act as a catalyst for physically orchemically binding the photosensitizing material and the electrontransport material to a surface of an inorganic material, or maystoichiometrically act for advantageously transfer chemical equilibrium.

Further, a condensation auxiliary agent, such as a thiol and a hydroxycompound, may be used in combination.

Specific examples of solvents for dissolving or dispersing thephotosensitizing material include, but are not limited to, water;alcohol solvents such as methanol, ethanol, and isopropyl alcohol;ketone solvents such as acetone, methyl ethyl ketone, and methylisobutyl ketone; ester solvents such as ethyl formate, ethyl acetate,and n-butyl acetate; ether solvents such as diethyl ether,dimethoxyethane, tetrahydrofuran, dioxolan, and dioxane; amide solventssuch as N,N-dimethylformamide, N,N-dimethylacetamide, andN-methyl-2-pyrrolidone; halogenated hydrocarbon solvents such asdichloromethane, chloroform, bromoform, methyl iodide, dichloroethane,trichloroethane, trichloroethylene, chlorobenzene, o-dichlorobenzene,fluorobenzene, bromobenzene, iodobenzene, and 1-chloronaphthalene; andhydrocarbon solvents such as n-pentane, n-hexane, n-octane,1,5-hexadiene, cyclohexane, methylcyclohexane, cyclohexadiene, benzene,toluene, o-xylene, m-xylene, p-xylene, ethylbenzene, and cumene. Thesesolvents can be used alone or in combination with others.

Some photosensitizing materials more effectively work when aggregationis suppressed. Therefore, an aggregation dissociating agent can be usedin combination.

Specific preferred examples of the aggregation dissociating agentinclude, but are not limited to, steroid compounds such as cholic acidand chenodeoxycholic acid; long-chain alkylcarboxylic acids; andlong-chain alkylsulfonic acids.

The addition amount of the aggregation dissociating agent is preferablyfrom 0.01 to 500 parts by mass, more preferably from 0.1 to 100 parts bymass, based on 1 part by mass of the photosensitizing material.

Preferably, the temperature at the adsorption of the photosensitizingmaterial alone or a combination of the photosensitizing material andaggregation dissociating agent is in the range of from −50 to 200° C.The adsorption may be performed under either static condition orstirring.

The stirring may be performed by, for example, a stirrer, a ball mill, apaint conditioner, a sand mill, an attritor, a disperser, or anultrasonic disperser.

The time required for the adsorption is, preferably, from 5 seconds to1,000 hours, more preferably from 10 seconds to 500 hours, and mostpreferably from 1 minute to 150 hours. Preferably, the adsorption isperformed in dark place.

Hole Transport Layer

The hole transport layer 6 may comprise an electrolytic solution inwhich a redox pair is dissolved in an organic solvent, a gel electrolytein which an organic solvent solution of a redox pair is impregnated in apolymer matrix, a molten salt containing a redox pair, a solidelectrolyte, an inorganic hole transport material, and/or an organichole transport material. Among these, organic hole transport materialsare preferable.

In the following descriptions, the hole transport layer 6 may bedescribed as comprising an organic hole transport material, for thepurpose of illustration and not limitation.

The hole transport layer 6 may have either a single-layer structurecomprising a single material or a multi-layer structure comprisingmultiple types of materials. In a case in which the hole transport layer6 has a multi-layer structure, it is preferable that a hole transportlayer disposed close to the second electrode 7 contains a polymericmaterial. By using the polymeric material having high film-formingperformance, the surface of the porous electron transport layer 4 can besmoothened and thereby photoelectric conversion property can beimproved.

In addition, because the polymeric material hardly permeates the porouselectron transport layer 4, the surface of the porous electron transportlayer 4 can be sufficiently covered with the polymeric material andthereby the occurrence of short circuit is prevented and improvedperformance is provided.

In a case in which the hole transport layer 6 has a single-layerstructure, the hole transport layer 6 may contain a known organic holetransport compound as the organic hole transport material.

Specific examples of such compounds include, but are not limited to,oxadiazole compounds described in JP-34-5466-B; triphenylmethanecompounds described in JP-45-555-B; pyrazoline compounds described inJP-52-4188-B; hydrazone compounds described in JP-55-42380-B; oxadiazolecompounds described in JP-56-123544-A; tetraarylbenzidine compoundsdescribed in JP-54-58445-A; and stilbene compounds described inJP-58-65440-A and JP-60-98437-A.

Specific preferred examples of the organic hole transport materialinclude a material represented by the following formula (3). Preferably,the content rate of the organic hole transport material represented bythe formula (3) in the hole transport layer 6 is from 30% to 99% bymass, and more preferably from 65% to 80% by mass.

In the formula (3), R₃ represents hydrogen atom or methyl group.

In particular, a hole transport material represented by the formula (3)when R₃ representing hydrogen atom, that is2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamino)-9,9′-spirobifluorene,also known as spiro-OMeTAD, described in Adv. Mater., 813, vol. 17,2005, is preferable, and another hole transport material represented bythe formula (3) when R₃ representing methyl group, that is HTM1described in ACS Appl. Mater. Interfaces, 2015, 7(21), pp. 11107-11116,is more preferable.

The spiro-OMeTAD has a high hole drift mobility and a molecularstructure in which two benzidine backbone molecules are twisted andbound. Therefore, the spiro-OMeTAD forms an electron cloud having anearly spherical shape and exhibits excellent photoelectric conversionproperty due to excellent hopping conductivity between molecules. Inaddition, the spiro-OMeTAD is highly soluble in various organicsolvents. Being amorphous (i.e., having no crystalline structure), thespiro-OMeTAD can be densely packed in the porous electron transportlayer 4, which is an advantageous property for solid-statedye-sensitizing solar cells. Furthermore, having no light-absorbingproperty at wavelengths equal to and greater than 450 nm, thespiro-OMeTAD can allow the photosensitizing material to effectivelyabsorb light, which is also an advantageous property for solid-statedye-sensitizing solar cells.

The HTM1 has an advantageous property such that crystallization can besuppressed under high-temperature environments by increasing sterichindrance by introducing an alkyl group to the molecular backbonethereof.

Preferred examples of the inorganic hole transport material includemetal complex salts. A metal complex salt generally comprises a metalcation, a ligand, and an anion. Specific examples of the metal complexsalt include all possible combinations of metal cations, ligands, andanions exemplified below. Specific examples of the metal cation in themetal complex salt include, but are not limited to, cations of chromium,manganese, iron, cobalt, nickel, copper, molybdenum, ruthenium, rhodium,palladium, silver, tungsten, rhenium, osmium, iridium, gold, andplatinum. Among these, cations of chromium, iron, nickel, and copper arepreferable.

Specific examples of the ligand for forming the metal complex saltinclude, but are not limited to, the following compounds (A-01) to(A-28). Each of these compounds can be used alone or in combination withothers.

Specific examples of the anion in the metal complex salt include, butare not limited to, hydride ion (H⁻), fluoride ion (F⁻), chloride ion(Cl⁻), bromide ion (Br⁻), iodide ion (I⁻), hydroxide ion (OH⁻), cyanideion (CN⁻), nitrate ion (NO₃ ⁻), nitrite ion (NO₂ ⁻), hypochlorite ion(ClO⁻), chlorite ion (ClO₂ ⁻), chlorate ion (ClO₃ ⁻), perchlorate ion(ClO₄ ⁻), permanganate ion (MnO₄ ⁻), acetate ion (CH₃COO⁻), hydrogencarbonate ion (HCO₃ ⁻), dihydrogen phosphate ion (H₂PO₄ ⁻), hydrogensulfate ion (HSO₄ ⁻), hydrogen sulfide ion (HS⁻), thiocyanate ion(SCN⁻), tetrafluoroborate ion (BF₄ ⁻), hexafluorophosphate ion (PF₆ ⁻),tetracyanoborate ion (B(CN)₄ ⁻), dicyanoamine ion (N(CN)₂ ⁻),p-toluenesulfonate ion (TsO⁻), trifluoromethylsulfonate ion (CF₃SO₂ ⁻),bis(trifluoromethylsulfonyl)amine ion (N(SO₂CF₃)₂ ⁻),tetrahydroxoaluminate ion ([AL(OH)₄]⁻ or [Al(OH)₄(H₂O)₂]⁻),dicyanoargentate(I) ion ([Ag(CN)₂]⁻), tetrahydroxochromate(III) ion([Cr(OH)₄]⁻), tetrachloroaurate(III) ion ([AuCl₄]⁻), oxide ion (O₂ ⁻),sulfide ion (S₂ ⁻), peroxide ion (O₂ ²⁻), sulfate ion (SO₄ ²⁻), sulfiteion (SO₃ ²⁻), thiosulfate ion (S₂O₃ ²⁻), carbonate ion (CO₃ ²⁻),chromate ion (CrO₄ ²⁻), dichromate ion (Cr₂O₇ ²⁻), hydrogen phosphateion (HPO₄ ²⁻), tetrahydroxozincate(II) ion ([Zn(OH)₄]²⁻),tetracyanozincate(II) ion ([Zn(CN)₄]²⁻), tetrachlorocuprate(II) ion([CuCl₄]²⁻), phosphate ion, (PO₄ ³⁻), hexacyanoferrate(III) ion([Fe(CN)₆]³⁻), bis(thiosulfato)argentate(I) ion ([Ag(S₂O₃)₂]³⁻), andhexacyanoferrate(II) ion ([Fe(CN)₆]⁴⁻).

Among these anions, tetrafluoroborate ion, hexafluorophosphate ion,tetracyanoborate ion, bis(trifluoromethylsulfonyl)amine ion, andperchlorate ion are preferable.

Each of the metal complex salts can be used alone or in combination withothers. In particular, cobalt complex salts comprisingtris-(2,2′-bipyridine)cobalt(II)di(bis(trifluoromethane)sulfonimide)(available from Dyenamo as a product name DN-C13) andtris-(2,2′-bipyridine)cobalt(III)tri(bis(trifluoromethane)sulfonimide)(available from Dyenamo as a product name DN-C14), and copper metalsalts comprising bis-(2,9-dimethyl-1,10-phenanthroline)copper(I)bis(trifluoromethanesulfonyl)imide (available from Dyenamo as a productname DN-Cu01) andbis-(2,9-dimethyl-1,10-phenanthroline)copper(II)bis(trifluoromethanesulfonyl)imidechloride (available from Dyenamo as a product name DN-Cu02) arepreferable. Preferably, the content rate of the inorganic hole transportmaterial in the hole transport layer 6 is from 30% to 80% by mass, andmore preferably from 50% to 70% by mass.

Preferably, the hole transport layer 6 has a configuration such that thehole transport layer 6 is entering pores of the porous electrontransport layer 4. The average thickness of the hole transport layer 6on the electron transport layer 4 is preferably 0.01 μm or more, morepreferably from 0.1 to 10 μm.

In a case in which the hole transport layer 6 has a multi-layerstructure, the hole transport layer disposed close to the secondelectrode 7 preferably contains a polymeric material. Examples of thepolymeric material include known hole transport polymeric materials.

Specific examples of such hole transport polymeric materials include,but are not limited to, polythiophene compounds such aspoly(3-n-hexylthiophene), poly(3-n-octyloxythiophene),poly(9,9′-dioctyl-fluorene-co-bithiophene),poly(3,3′″-didodecyl-quarter-thiophene),poly(3,6-dioctylthieno[3,2-b]thiophene),poly(2,5-bis(3-decylthiophene-2-yl)thieno[3,2-b]thiophene),poly(3,4-didecylthiophene-co-thieno[3,2-b]thiophene),poly(3,6-dioctylthieno[3,2-b]thiophene-co-thieno[3,2-b]thiophene),poly(3,6-dioctylthieno[3,2-b]thiophene-co-thiophene), andpoly(3,6-dioctylthieno[3,2-b]thiophene-co-bithiophene);polyphenylenevinylene compounds such aspoly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene],poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene], andpoly[2-methoxy-5-(2-ethylphenyloxy)-1,4-phenylenevinylene)-co-(4,4′-biphenylene-vinylene);polyfluorene compounds such as poly(9,9′-didodecylfluorenyl-2,7-diyl),poly[(9,9-dioctyl-2,7-divinylenefluorene)-alt-co-(9,10-anthracene)],poly[(9,9-dioctyl-2,7-divinylenefluorene)-alt-co-(4,4′-biphenylene)],poly[(9,9-dioctyl-2,7-divinylenefluorene)-alt-co-(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene)],and poly[(9,9-dioctyl-2,7-diyl)-co-(1,4-(2,5-dihexyloxy)benzene)];polyphenylene compounds such as poly[2,5-dioctyloxy-1,4-phenylene] andpoly[2,5-di(2-ethylhexyloxy-1,4-phenylene]; polyarylamine compounds suchaspoly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(N,N′-diphenyl)-N,N′-di(p-hexylphenyl)-1,4-diaminobenzene],poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(N,N′-bis(4-octyloxyphenyl)benzidine-N,N′-(1,4-diphenylene)],poly[(N,N′-bis(4-octyloxyphenyl)benzidine-N,N′-(1,4-diphenylene)],poly[(N,N′-bis(4-(2-ethylhexyloxy)phenyl)benzidine-N,N′-(1,4-diphenylene)],poly[phenylimino-1,4-phenylenevinylene-2,5-dioctyloxy-1,4-phenylenevinylene-1,4-phenylene],poly[p-tolylimino-1,4-phenylenevinylene-2,5-di(2-ethylhexyloxy)-1,4-phenylenevinylene-1,4-phenylene],and poly[4-(2-ethylhexyloxy)phenylimino-1,4-biphenylene]; andpolythiadiazole compounds such aspoly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(1,4-benzo(2,1′,3)thiadiazole]and poly(3,4-didecylthiophene-co-(1,4-benzo(2,1′,3)thiadiazole).

Among these materials, polythiophene compounds and polyarylaminecompounds are preferable for carrier mobility and ionization potential.

An additive may be further added to the organic hole transport material.

Specific examples of the additive include, but are not limited to,iodine; metal iodides such as lithium iodide, sodium iodide, potassiumiodide, cesium iodide, calcium iodide, copper iodide, iron iodide, andsilver iodide; iodine salts of quaternary ammonium compounds such astetraalkylammonium iodide and pyridinium iodide; metal bromides such aslithium bromide, sodium bromide, potassium bromide, cesium bromide, andcalcium bromide; bromine salts of quaternary ammonium compounds such astetraalkylammonium bromide and pyridinium bromide; metal chlorides suchas copper chloride and silver chloride; metal acetates such as copperacetate, silver acetate, and palladium acetate; metal sulfates such ascopper sulfate and zinc sulfate; metal complexes such asferrocyanate-ferricyanate and ferrocene-ferricinium ion; sulfurcompounds such as sodium polysulfide and alkyl thiol-alkyl disulfide;viologen dye; hydroquinones; ion liquids described in Inorg. Chem. 35(1996) 1168 such as 1,2-dimethyl-3-n-propyl imidazolinium iodide,1-methyl-3-n-hexyl imidazolinium iodide, 1,2-dimethyl-3-ethylimidazolium trifluoromethane sulfonate, 1-methyl-3-butyl imidazoliumnonafluorobutyl sulfonate, and 1-methyl-3-ethyl imidazoliumbis(trifluoromethyl) sulfonylimide; basic compounds such as pyridine,4-t-butyl pyridine, and benzimidazole; and lithium compounds such aslithium trifluoromethane sulfonylimide and lithium diisopropylimide.

It is considered that, in the hole transport layer 6, the basic compoundA is mainly present near the interface between the electron transportlayer 4 and the hole transport layer 6 and suppresses reverse electrontransfer from the electron transport layer 4 (i.e., electron transferfrom the electron transport layer 4 to the hole transport layer 6). Itis also considered that the cation in the ionic compound B is mainlypresent near the interface between the electron transport layer 4 andthe hole transport layer 6, and the anion in the ionic compound B isdoped in the hole transport layer 6. Under high-temperatureenvironments, preferably, not only crystallization of the hole transportlayer 6 itself but also the contact interface between the hole transportlayer 6 and the electron transport layer 4 (and the photosensitizingcompound 5) are properly controlled. This is because voltage reductionmay be caused when short circuit or resistance increase occurs at thecontact interface and/or a defective hole conducting path generates dueto crystallization.

In the present embodiment, the basic compound A represented by thefollowing formula (1) and the ionic compound B represented by thefollowing formula (2) are added to the hole transport layer 6. Due tothis configuration, the internal resistance of the photoelectricconversion element becomes much higher and therefore current loss underweak light (such as indoor light) is reduced and the open voltagebecomes higher. As a result, under high-temperature environments,crystallization of the hole transport layer 6 can be suppressed and thecontact interface between the electron transport layer 4 and the holetransport layer 6 can be properly maintained, thus advantageouslysuppressing voltage reduction.

In the formula (1), each of R₁ and R₂ independently represents an alkylgroup or an aromatic hydrocarbon group, or R₁ and R₂ share bondconnectivity to form a nitrogen-containing heterocyclic ring.

Specific examples of the basic compound A represented by the formula (1)include, but are not limited to, the following compounds represented bythe respective formulae (1-1) to (1-9).

The combination of alphabets and numerals attached to each structuralformula, if any, represents the compound number defined in the chemicalsubstance database “Japan Chemical Substance Dictionary” created byJapan Science and Technology Agency.

The compound represented by the formula (1-1) itself is conventionallyknown. It is also known that some of the compounds represented by theformula (1) are used as a basic compound for liquid-state dye-sensitizedsolar cells using an iodine electrolyte.

However, it is known that when the above basic compound is used for theconventional liquid-state dye-sensitized solar cell using an iodineelectrolyte, the short-circuit current density is notably reduced, whilethe open voltage is maintained, thereby significantly degradephotoelectric conversion property.

On the other hand, when the basic compound A represented by the formula(1) is used for a solid-state dye-sensitized solar cell comprising theabove organic hole transport material in the hole transport layer 6, theamount of decrease in short-circuit current density can be reduced andthe open voltage can be increased, thereby providing excellentphotoelectric conversion property. Such a solar cell has a distinctiveadvantage in performing photoelectric conversion under weak light, suchas indoor light.

Preferably, the content of the basic compound A represented by theformula (1) in the hole transport layer 6 is from 1 to 20 parts by mass,more preferably from 5 to 15 parts by mass, based on 100 parts by massof the organic hole transport material.

The hole transport layer 6 further contains the ionic compound Brepresented by the following formula (2).

In the formula (2), X⁺ represents a counter cation.

Preferred examples of X in the ionic compound B represented by theformula (2) include, but are not limited to, nitrogen-containingheterocyclic compounds (e.g., imidazolium and pyrrolidinium) and alkalimetals (e.g., Li, Na, and K). In particular, Li is most preferable.

Specific preferred examples of the ionic compound B represented by theformula (2) include lithium(fluorosulfonyl)(trifluoromethanesulfonyl)imide (also known asLi-FTFST). Lithium (fluorosulfonylX)(trifluoromethanesulfonyl)imide is aknown compound (described in J. Phys. Chem, C 2013, 117, 24206-24212),but any application of this compound to solar cells has not beenreported.

Even when the ionic compound B represented by the formula (2) is used,the contact interface between the electron transport layer 4 and thehole transport layer 6 cannot be sufficiently controlled and theoccurrence of short circuit and resistance increase cannot besufficiently suppressed under high-temperature environments, unless thebasic compound A represented by the formula (1) is used in combination.

When the basic compound A represented by the formula (1) and the ioniccompound B represented by the formula (2) are used in combination,crystallization of the hole transport layer 6 can be suppressed and thecontact interface between the electron transport layer 4 and the holetransport layer 6 can be properly maintained, thus suppressing voltagereduction.

The molar ratio (A:B) between the basic compound A and the ioniccompound B in the hole transport layer 6 is from 20:1 to 10:10, and morepreferably from 10:1 to 10:4.

For the purpose of improving conductivity, an oxidant may be added forconverting a part of the organic hole transport material into radicalcations.

Specific examples of the oxidant include, but are not limited to,tris(4-bromophenyl)aminium hexachloroantimonate, silverhexafluoroantimonate, nitrosonium tetrafluoroborate, silver nitrate, andcobalt complex compounds.

Not all the organic hole transport materials need to be oxidized by theoxidant and only a part of them may be oxidized. The oxidant having beenadded to the system may be either taken out or kept therein.

The organic hole transport layer may be directly formed on the electrontransport layer 4 that is carrying the photosensitizing material. Theorganic hole transport layer is not limited in production method and canbe produced by, for example, a method of forming a thin layer in vacuum,such as vacuum deposition, or a wet film-forming method. For reducingproduction cost, a wet film-forming method in which a coating liquid isapplied onto the electron transport layer 4 is preferable.

In this wet film-forming method, how to apply the coating liquid is notparticularly limited. For example, the coating liquid may be applied bymeans of dipping, spraying, wire bar, spin coating, roller coating,blade coating, gravure coating, or wet printing such as relief, offset,gravure, intaglio, rubber plate, and screen printings. Alternatively,the layer may be formed in a supercritical fluid or a subcritical fluidhaving lower temperature and pressure than the critical point.

The supercritical fluid is not limited in substance so long as it existsas a non-cohesive high-density fluid at temperatures and pressuresbeyond the region where gases and liquids can coexist (i.e., thecritical point), without cohering even under compression, while having atemperature equal to or above the critical temperature and a pressureequal to or above the critical pressure. Specifically, those having alow critical temperature are preferable.

Specific examples of the supercritical fluid include, but are notlimited to, carbon monoxide, carbon dioxide, ammonia, nitrogen, water,alcohol solvents (e.g., methanol, ethanol, and n-butanol), hydrocarbonsolvents (e.g., ethane, propane, 2,3-dimethylbutane, benzene, andtoluene), halogen solvents (e.g., methylene chloride andchlorotrifluoromethane), and ether solvents (e.g., dimethyl ether).Among these substances, carbon dioxide, having a supercritical pressureof 7.3 MPa and a supercritical temperature of 31° C., is preferable,because carbon dioxide is easy to put into a supercritical state andeasy to handle owing to its non-combustibility.

Each of these fluids can be used alone or in combination with others.

The subcritical fluid is not limited in substance so long as it existsas a high-pressure liquid at temperatures and pressures near thecritical point.

The above-described substances preferable for the supercritical fluidare also preferable for the subcritical fluid.

The supercritical fluid is not limited in critical temperature andcritical pressure, but preferably has a critical temperature of from−273° C. to 300° C., more preferably from 0° C. to 200° C.

In addition, an organic solvent and/or entrainer can be used incombination with the supercritical fluid or subcritical fluid. Additionof an organic solvent and/or entrainer facilitates adjustment ofsolubility in the supercritical fluid.

Specific examples of the organic solvent include, but are not limitedto, ketone solvents such as acetone, methyl ethyl ketone, and methylisobutyl ketone; ester solvents such as ethyl formate, ethyl acetate,and n-butyl acetate; ether solvents such as diisopropyl ether,dimethoxyethane, tetrahydrofuran, dioxolan, and dioxane; amide solventssuch as N,N-dimethylformamide, N,N-dimethylacetamide, andN-methyl-2-pyrrolidone; halogenated hydrocarbon solvents such asdichloromethane, chloroform, bromoform, methyl iodide, dichloroethane,trichloroethane, trichloroethylene, chlorobenzene, o-dichlorobenzene,fluorobenzene, bromobenzene, iodobenzene, and 1-chloronaphthalene; andhydrocarbon solvents such as n-pentane, n-hexane, n-octane,1,5-hexadiene, cyclohexane, methylcyclohexane, cyclohexadiene, benzene,toluene, o-xylene, m-xylene, p-xylene, ethylbenzene, and cumene.

After the organic hole transport material is provided on the firstelectrode 2 having the electron transport material covered with thephotosensitizing material thereon, a press processing may be conducted.The press processing brings the organic hole transport material into amore intimate contact with the porous electrode, thus improvingefficiency.

The press processing may be a press molding using a flat plate, such asan IR tablet pelletizer, or a roll press using a roller. The pressure inthe press processing is preferably 10 kgf/cm² or more, and morepreferably 30 kgf/cm² or more. The pressing time is preferably within 1hour. Heat can be applied during the press processing, if necessary.

It is possible that a release material is sandwiched between the presserand the electrode in the press processing.

Specific examples of the release material include, but are not limitedto, fluorine resins such as polytetrafluoroethylene,polychlorotrifluoroethylene, tetrafluoroethylene-hexafluoropropylenecopolymer, perfluoroalkoxyfluoro resin, polyvinylidene fluoride,ethylene-tetrafluoroethylene copolymer, ethylene-chlorotrifluoroethylenecopolymer, and polyvinyl fluoride.

After the press processing and before provision of an oppositeelectrode, a metal oxide may be provided between the organic holetransport material and the second electrode. Specific examples of themetal oxide include, but are not limited to, molybdenum oxide, tungstenoxide, vanadium oxide, and nickel oxide. Among these materials,molybdenum oxide is preferable.

There is no limit on how to provide the metal oxide on the organic holetransport material. For example, a method of forming a thin layer invacuum, such as sputtering and vacuum deposition, and a wet film-formingmethod can be employed.

Specifically, a wet film-forming method in which a paste dispersing apowder or sol of a metal oxide is applied to the hole transport layer 6is preferable.

In this wet film-forming method, how to apply the paste is notparticularly limited. For example, the paste may be applied by means ofdipping, spraying, wire bar, spin coating, roller coating, bladecoating, gravure coating, or wet printing such as relief, offset,gravure, intaglio, rubber plate, and screen printings.

The film thickness is preferably from 0.1 to 50 nm and more preferablyfrom 1 to 10 nm.

Second Electrode

The second electrode 7 is formed on the hole transport layer 6 or themetal oxide described above.

The second electrode 7 may have a similar configuration to the firstelectrode. However, the substrate is not necessary so long as thestrength and sealing performance are sufficiently secured.

Specific examples of usable materials for the second electrode 7include, but are not limited to, metals such as platinum, gold, silver,copper, and aluminum; carbon compounds such as graphite, fullerene,carbon nanotube, and graphene; conductive metal oxides such as ITO, FTO,and ATO; and conductive polymers such as polythiophene and polyaniline.

The second electrode 7 is not limited in thickness. The second electrode7 may be formed of a single material or a mixture of two or morematerials.

The second electrode 7 can be formed on the hole transport layer 6 bymeans of, for example, coating, lamination, vapor deposition, CVD(chemical vapor deposition), or bonding, depending on the types ofmaterials constituting the second electrode 7 and the hole transportlayer 6.

To function as a photoelectric conversion element, preferably, at leastone of the first electrode 2 and the second electrode 7 is substantivelytransparent. In the present embodiment, preferably, the first electrode2 is transparent to allow solar light to enter from the first electrode2 side. In this case, the second electrode 7 is preferably made of alight reflective material such as metal-deposited orconductive-oxide-deposited glass or plastic, or a metallic thin film.

It is also effective to provide an antireflective layer on the solarlight incoming side.

Use Application

The photoelectric conversion element in accordance with some embodimentsof the present invention refers to an element that converts opticalenergy into electric energy or an element that converts electric energyinto optical energy. The photoelectric conversion element may be usedfor solar cells and photodiodes. In particular, the photoelectricconversion element is preferably used for solar cells.

The photoelectric conversion element in accordance with some embodimentsof the present invention is applicable to power-supply devices when usedin combination with a circuit board that controls a generated current.Specific examples of instruments using such a power-supply deviceinclude, but are not limited to, electronic desk calculators andwatches. In addition, such a power-supply device using the photoelectricconversion element in accordance with some embodiments of the presentinvention is applicable to cell phones, electronic organizers, andelectronic papers. In addition, such a power-supply device using thephotoelectric conversion element in accordance with some embodiments ofthe present invention can also be used as an auxiliary power supply forlengthening the continuous operating time of charging-type orbattery-type electronic devices. Furthermore, the photoelectricconversion element can be used as a substitute of a primary battery thatis combined with a secondary battery, as a stand-alone power supply forsensors.

EXAMPLES

Further understanding can be obtained by reference to certain specificexamples which are provided herein for the purpose of illustration onlyand are not intended to be limiting.

Example 1 Preparation of Titanium Oxide Semiconductor Electrode(Electron Transport Layer)

A dense hole blocking layer 3 was formed with titanium oxide on an ITOglass substrate, in which an ITO conductive film serving as the firstelectrode 2 and a glass substrate serving as the substrate 1 areintegrated, by reactive sputtering of oxygen gas using a titanium metaltarget.

The first electrode 2 was laser-etched by a laser device to obtain a10-cell serial substrate.

Next, 3 g of titanium oxide (P90 available from Nippon Aerosil Co.,Ltd.), 0.2 g of acetylacetone, and 0.3 g of a surfactant(polyoxyethylene octyl phenyl ether, available from Wako Pure ChemicalIndustries, Ltd.) were subjected to a bead mill treatment, along with5.5 g of water and 1.0 g of ethanol, for 12 hours, thus obtaining atitanium oxide dispersion liquid. The resulting titanium oxidedispersion liquid was mixed with 1.2 g of a polyethylene glycol (#20,000available from Wako Pure Chemical Industries, Ltd.) to prepare a paste.

The paste was applied onto the hole blocking layer 3 so as to have anaverage thickness of 1.5 μm, dried at room temperature, and then burntin the air at 500° C. for 30 minutes, thus forming a porous electrontransport layer 4. As a result, a titanium oxide semiconductor electrodewas prepared.

Preparation of Photoelectric Conversion Element

The titanium oxide semiconductor electrode prepared above was dipped ina 0.5 mM acetonitrile/t-butanol (1/1 by volume) solution of the compoundD358 (available from Mitsubishi Paper Mills Limited) represented by theformula (iii) and left to stand for 1 hour in a dark place, so that thephotosensitizing compound 5 was adsorbed thereto.

Next, 22.38 mg (150 mM) of the basic compound A represented by theformula (1-3) (i.e., 4-pyrrolidinopyridine available from Tokyo ChemicalIndustry Co., Ltd.) and 9.62 mg (40 mM) of the ionic compound B (i.e.,lithium (fluorosulfonyl)(trifluoromethanesulfonyl)imide, also known asLi-FTFSI, available from Provisco CS) were added to 1 mL of achlorobenzene solution of 183.8 mg of an organic hole transport materialrepresented by the following formula (4) (i.e.,9,9′-([1,1′-biphenyl]-4,4′-diyl)bis(N3,N3,N6,N6-tetrakis(4-methoxyphenyl)-9H-carbazole-3,6-diamine),also known as X51, available from Dyenamo). Thus, a hole transport layercoating liquid was prepared having a molar ratio (A:B) of 10:2.67.

Next, the semiconductor electrode carrying the photosensitizing compoundwas coated with the hole transport layer coating liquid by spin coating,thus forming the hole transport layer 6. As a result, a photoelectricconversion layer was formed.

Next, a silver layer having a thickness of 100 nm was vacuum-depositedon the photoelectric conversion layer using a mask having a 6-cellserial pattern to form the second electrode 7. Thus, a photoelectricconversion element was prepared.

Evaluation of Photoelectric Conversion Element

The above-prepared photoelectric conversion element was connected to asecondary battery charging circuit illustrated in FIG. 2. Referring toFIG. 2, a PMIC (power management integrated circuit) 12 receives poweroutput from a photoelectric conversion element 11 and outputs stabilizedpower required by an external circuit 13. Numerals 121, 122, and 123respectively denote an environmental power input terminal, a batteryterminal, and a stabilized power output terminal. The circuit 13 is putinto operation by the power output from the PMIC 12. The circuit 13comprises a sensor and a communication circuit. A numeral 131 denote apower input terminal. A secondary battery 14 is charged by environmentalpower while the PMIC 12 is outputting a specifically stabilized power.

As the PMIC 12, LTC3331 available from Linear Technology Corporation wasused. The output voltage of the PMIC 12 was set to 3.0 V, and thelockout voltage of the PMIC 12 was set to 45 V (at rising) and 4 V (atfalling). While the PMIC 12 was outputting a voltage of 3.0 V±3% and thephotoelectric conversion element 11 was outputting a voltage of 4.0 V orgreater, the secondary battery was charged.

Whether the secondary battery 14 had been charged or not was determinedby confirming a voltage increase by monitoring the voltage of thesecondary battery 14 with a data logger (midi LOGGER GL900 availablefrom Graphtec Corporation).

Secondary battery charing ability was evaluated by connecting thephotoelectric conversion element to the circuit and left to to stand for500 hours under high-temperature (100° C.) environments and white LEDirradiation (2,000 Lux, 0.48 mW/cm²).

Evaluation Criteria

-   -   A: Capable of charging. Voltage was 4.5 V or greater.    -   B: Capable of charging.    -   C: Incapable of charging.

As a photoelectric conversion property, open voltage V_(OC) was alsoevaluated. The evaluation results are shown in Table 1.

In measuring the photoelectric conversion property (open voltageV_(OC)), an LED desk lamp (CDS-90α available from Cosmotechno Co., Ltd.in Study Mode) having high color rendering property was used as a whiteLED lamp, and a solar cell evaluating system (As-510-PV03 available fromNF Corporation) was used as an evaluation device. The evaluation resultsmeasured before the photoelectric conversion element was heated are alsoshown in Table 1.

Example 2

The procedure in Example 1 was repeated except for replacing the holetransport material with another organic hole transport materialrepresented by the following formula (5) (i.e.,2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamino)-9,9′-spirobifluorene,also known as SHT-263, available from Merck). The evaluation results areshown in Table 1.

Example 3

The procedure in Example 1 was repeated except for replacing the holetransport material with another organic hole transport materialrepresented by the following formula (6) (i.e., LT-S9170, also known asHTM1, available from Luminescence Technology Corp.).

The evaluation results are shown in Table 1.

Examples 4 to 10

The procedure in Example 1 was repeated except for changing the molarratio (A:B) between the basic compound A and the ionic compound B asaccording to Table 1. The evaluation results are shown in Table 1.

Example 11 Preparation of Photoelectric Conversion Element

The titanium oxide semiconductor electrode prepared in Example 1 wasdipped in a 0.5 mM acetonitrile/t-butanol (1/1 by volume) solution ofthe photosensitizing material D358 (available from Mitsubishi PaperMills Limited) represented by the formula (iii) and left to stand for 1hour in a dark place, so that the photosensitizing material was adsorbedthereto.

The semiconductor electrode carrying the photosensitizing material wascoated with 1 ml of a chlorobenzene solution of 152 mg (200 mM) ofbis-(2,9-dimethyl-1,10-phenanthroline)copper(I)bis(trifluoromethanesulfonyl)imide (available from Dyenamo as a productname DN-Cu01), 39.8 mg (50 mM) ofbis-(2,9-dimethyl-1,10-phenanthroline)copper(II)bis(trifluoromethanesulfonyl)imidechloride (available from Dyenamo as a product name DN-Cu02), 74.6 mg(500 mM) of the basic compound A represented by the formula (1-3) (i.e.,4-pyrrolidinopyridine available from Tokyo Chemical Industry Co., Ltd.),and 24.05 mg (100 mM) of the ionic compound B (i.e., lithium(fluorosulfonyl)(trifluoromethanesulfonyl)imide available from ProviscoCS), by spin coating. Further, a silver film having a thickness of about100 nm was formed thereon by vacuum vapor deposition. Thus, asolid-state dye-sensitized solar cell element was prepared.

Comparative Example 1

The procedure in Example 1 was repeated except for replacing the basiccompound A with 4-tertiary-butyl pyridine (TBP available from TokyoChemical Industry Co., Ltd.). The evaluation results are shown in Table1.

Comparative Example 2

The procedure in Example 1 was repeated except for replacing the ioniccompound B with lithium bis(trifluoromethanesulfonyl)imide (also knownas Li-TFSI, available from Kanto Chemical Co., Inc.). The evaluationresults are shown in Table 1.

Comparative Example 3

The procedure in Example 1 was repeated except for replacing the ioniccompound B with lithium bis(fluorosulfonyl)imide (also known as Li-FSI,available from KISHIDA CHEMICAL Co., Ltd.). The evaluation results areshown in Table 1.

Comparative Example 4

The procedure in Example 1 was repeated except for replacing the ioniccompound B with a mixture of lithium bis(trifluoromethanesulfonyl)imide(also known as Li-TFSI, available from Kanto Chemical Co., Inc.) andlithium bis(fluorosulfonyl)imide (also known as Li-FSI, available fromKISHIDA CHEMICAL Co., Ltd.). The evaluation results are shown in Table1.

TABLE 1 Hole Molar Initial Stage After Heating at 100° C. Basic IonicTransport Ratio Charging Decrease Charging Compound A Compound BMaterial (A:B) V_(OC) Ability V_(OC) Rate (%) Ability Example 1 (1-3)Li-FTFSI X51 10:2.67 5.28 A 4.32 18.2 B Example 2 (1-3) Li-FTFSI SHT-26310:2.67 5.52 A 4.68 15.2 A Example 3 (1-3) Li-FTFSI HTM1 10:2.67 5.58 A4.86 12.9 A Example 4 (1-1) Li-FTFSI HTM1 10:2.67 5.46 A 4.83 11.5 AExample 5 (1-6) Li-FTFSI HTM1 10:2.67 5.40 A 4.89 9.4 A Example 6 (1-3)Li-FTFSI HTM1 10:0.8 5.94 A 4.47 24.7 B Example 7 (1-3) Li-FTFSI HTM110:1 5.88 A 4.92 16.3 A Example 8 (1-3) Li-FTFSI HTM1 10:2 5.82 A 4.8317.0 A Example 9 (1-3) Li-FTFSI HTM1 10:4 5.43 A 4.53 16.6 A Example 10(1-3) Li-FTFSI HTM1 10:5 5.22 A 4.14 20.7 B Example 11 (1-3) Li-FTFSIDN-Cu01/ 10:2 5.28 A 4.86 8.0 A DN-Cu02 Comparative TBP Li-FTFSI X5110:2.67 5.01 A 3.54 29.3 C Example 1 Comparative (1-3) Li-TFSI X5110:2.67 5.34 A 3.30 38.2 C Example 2 Comparative (1-3) Li-FSI X5110:2.67 5.10 A 3.72 27.1 C Example 3 Comparative (1-3) Li-TSFI/ X5110:2.67 5.28 A 3.66 30.7 C Example 4 Li-FSI

The results shown in Table 1 indicate that, in the photoelectricconversion elements of Examples 1 to 11, voltage reduction wassuppressed under high-temperature environments and secondary batterycharging ability was maintained. Therefore, these photoelectricconversion elements are usable without changing the settings of thePMIC. The reason for such results is considered that crystallization ofthe hole transport layer was suppressed and the contact interfacebetween the electron transport layer and the hole transport layer waskept in a good condition.

By contrast, in Comparative Examples 1 to 4 in which the molar ratio(A:B) between the basic compound A and the ionic compound B was out ofthe specified range, desired properties were not obtained.

It is clear from the evaluation results that the photoelectricconversion elements in accordance with some embodiments of the presentinvention are capable of suppressing voltage reduction and chargingsecondary battery regardless of environment temperature.

Numerous additional modifications and variations are possible in lightof the above teachings. It is therefore to be understood that, withinthe scope of the above teachings, the present disclosure may bepracticed otherwise than as specifically described herein. With someembodiments having thus been described, it will be obvious that the samemay be varied in many ways. Such variations are not to be regarded as adeparture from the scope of the present disclosure and appended claims,and all such modifications are intended to be included within the scopeof the present disclosure and appended claims.

1. A photoelectric conversion element comprising: a substrate; a firstelectrode; an electron transport layer comprising a photosensitizingcompound; a hole transport layer comprising: a basic compound Arepresented by the following formula (1):

where each of R₁ and R₂ independently represents an alkyl group or anaromatic hydrocarbon group, or R₁ and R₂ share bond connectivity to forma nitrogen-containing heterocyclic ring; and an ionic compound Brepresented by the following formula (2):

where X⁺ represents a counter cation; and a second electrode.
 2. Thephotoelectric conversion element of claim 1, further comprising a holeblocking layer.
 3. The photoelectric conversion element of claim 1,wherein the ionic compound B comprises lithium(fluorosulfonyl)(trifluoromethanesulfonyl)imide.
 4. The photoelectricconversion element of claim 1, wherein R₁ and R₂ share bond connectivityto form a nitrogen-containing heterocyclic ring.
 5. The photoelectricconversion element of claim 1, wherein a molar ratio (A:B) between ofthe basic compound A and the ionic compound B is from 10:1 to 10:4. 6.The photoelectric conversion element of claim 1, wherein the holetransport layer comprises an organic hole transport material representedby the following formula (3):

where R₃ represents hydrogen atom or methyl group.
 7. The photoelectricconversion element of claim 1, wherein the hole transport layer containsan inorganic hole transport material comprising a complex salt of ametal selected from the group consisting of cobalt, iron, nickel, andcopper.
 8. A solar cell comprising the photoelectric conversion elementof claim 1.